1. Field of the Invention
The present invention relates to a bypass unit for use, for example, in a thermal mass flowmeter sensor.
2. Description of the Related Art
It is well known that laminar flow conditions need to be maintained to measure the flow of a fluid in a shunt or bypass path. Attempts have been made to mount a variety of units to develop laminar flow. One such unit is disclosed, for example, in U.S. Pat. No. 4,522,058. The prior art unit comprises a tubular member, a solid core mounted coaxially within the tubular member, and a plurality of elongate straight wires extending between the tubular member and the core. The wires are likely to be bent when the core is press-fit into the tubular member. If this occurs, then pressure differentials between the inlet end and the outlet end of the unit are no longer proportional to the flow of a fluid resulting in measurement errors. Also, eddies are developed particularly when the wires are substantially bent. Consequently, the unit becomes impractical.
Eddies are hardly developed when a flow path, which is connected to the bypass path, is straight and long, or when a flow rate is low. A typical flow path is, however, curved immediately before it reaches a flowmeter. Such a curved flow path often causes measurement errors. To overcome this, a flow regulating filter in the form of a mesh is mounted to the inlet end of the bypass path. A problem with the use of the filter is that the inlet end of the bypass path is subject to substantial pressure loss.
The sensor has a limited resolution. Also, the sensor tube and the bypass path have limited flow rates. When a pressure differential .DELTA.P.sub.1 between the inlet and the outlet of the sensor tube and a flow rate Q.sub.1 of a fluid flowing through the sensor tube exceed a predetermined level, the former is no longer proportional to the latter resulting in measurement errors. Similarly, when a pressure differential .DELTA.P.sub.2 between the inlet and the outlet of the bypass path and a flow rate Q.sub.2 of a fluid both exceed a predetermined level, the former is no longer proportional to the latter resulting in measurement errors. Laminar flow can be developed when pressure differentials are proportional to the flow rate of a fluid. On the other hand, turbulence effects which cause measurement errors are created when a pressure differential is proportional to the square of the flow rate.
Generally, a laminar flow is defined as having a Reynolds number R.sub.D of less than 2320. Reynolds number R.sub. is represented as follows: EQU R.sub.D =(4Q.rho.)/(.pi.D.eta.) (1)
where Q is the volumetric flow rate of a fluid flowing through a tube, .rho. is the density of the fluid, .eta. is the viscosity of the fluid, and D is the diameter of the tube.
From equation (1), it is clear that an increase in the diameter D of the tube results in a corresponding increase in the volumetric flow rate Q of the fluid, provided that the Reynolds number R.sub.D remains constant. However, the sensor per se has a limited size, and the sensor tube and the bypass path also have limited diameters. A plurality of small diameter passages may be arranged in the bypass path in an attempt to substantially increase the diameter D of the tube. This requires a wide variety of elements to develop laminar flow so as to perform measurements over different flow ranges.
To solve the foregoing problems, attempts have been made to place a cylindrical core centrally within a bypass path as disclosed in Japanese laid-open utility model publication No. 59/72514. This arrangement eliminates the need for wires as used in U.S. Pat. No. 4,522,058 and develops no turbulence. A problem with this prior art arrangement is, however, that a fluid is supplied directly to the inlet end of a sensor tube upon introduction into the sensor tube. This causes the sensor tube to detect eddies and thus, generate undesirable noise. Another problem is that the prior art arrangement requires bypass paths of various sizes to perform measurements over increased flow ranges. In addition, the outer periphery of the core and the inner wall of the bypass path must finely be machined when the flow rate of a sensed fluid is less than 10 cc. This results in an increase in the manufacturing steps and costs.